Update CONTRIBUTING.md

Author: mcbarton

CONTRIBUTING.md

@@ -130,7 +130,7 @@ python package. This only needs to be enable once, and then you can execute the
 
 ```bash
 echo "Running test_xeus_cpp in Safari"
-emrun --no_browser --kill_exit --timeout 60 --browser-args="--headless --no-sandbox"  test_xeus_cpp.html &
+python $BUILD_PREFIX/bin/emrun.py --no_browser --kill_exit --timeout 60 --browser-args="--headless --no-sandbox"  test_xeus_cpp.html &
 python browser_tests_safari.py test_xeus_cpp.html
 ```
 
@@ -167,4 +167,4 @@ jupyter lite serve --XeusAddon.prefix=$PREFIX \
                    --contents notebooks/smallpt.ipynb \
                    --contents notebooks/images/marie.png \
                    --contents notebooks/audio/audio.wav
-```
+```